1. Field of the Invention
The present invention relates to an inverse telephoto large aperture wide angle lens, and more particularly to a focusing method thereof.
2. Related Background Art
Focusing is usually done in a photographic lens by moving the entire lens system. There are, however, used these days 35 mm single lens reflex cameras having an automatic focusing mechanism, and a focusing part thereof is desired to have a less weight to reduce a load on a lens drive section. An inverse telephoto large aperture wide angle lens has a substantial weight at a diverging lens group nearest to an object because of its structure, and it is desired to conduct the focusing by a part of lens groups excluding the diverging lens group.
There are well known wide angle lenses, which carry out the focusing by moving a part of lens system as described, for example as proposed in Japanese Patent Publication No. 54-30290, Japanese Laid-open Patent Application No. 57-35821, Japanese Laid-open Patent Application No. 62-249119, and Japanese Laid-open Patent Application No. 1-134411.
However, in the wide angle lens as disclosed in Japanese Patent Publication No. 54-30290, the focusing is conducted only by moving intermediate lens groups before the stop, and therefore asymmetry is likely to become greater between a coma below the principal ray and a coma above the principal ray. This causes a difficulty in application to the focusing of a large aperture lens with an f number of about 1.4. In contrast, in the wide angle lens as disclosed in Japanese Laid-open Patent Application No. 57-35821, the focusing is conducted by lens groups behind the stop, and therefore an air gap is necessary for focusing in the groups behind the stop. This causes a difficulty in assuring a sufficient back focus. Further, an effective aperture of the closest lens to an image surface must be increased in order to secure sufficient brightness of the edge of image field, which results in a difficulty in mounting of a large aperture lens with an f number of about 1.4 as an interchangeable lens into a camera body.
Also, the wide angle lens as disclosed in Japanese Laid-open Patent Application No. 62-249119 is suitable for a lens with an f number of about 2.8, but is not suitable for use as a large aperture lens with an f number of about 1.4 because of a large change of astigmatism upon focusing. The wide angle lens as disclosed in Japanese Laid-open Patent Application No. 1-134411 includes a compound lens with a weak refractive power disposed closest to the image field, which results in a difficulty to secure a sufficient back focus. Further, the compound lens closest to the image field is fixed, and therefore an effective aperture of the lens closest to the image field must be increased to prevent a lack of brightness of the edge of image field, which might be caused due to vignetting of peripheral optical fluxes in short range photography. There is a difficulty in mounting between the lens and the camera body in such a case.